1. Field of the Invention
This invention relates to amplifier apparatus including a radio frequency (RF) power transistor output stage and, more particularly, to such amplifier apparatus as operates at very high frequencies.
2. Description of the Related Art
Amplifier apparatus operating with RF power transistor output stages conventionally include automatic gain control (AGC) circuitry to maintain the output power substantially constant in the presence of variations in supply voltages, temperature, frequency, etc. Typically, direct voltages are monitored at various locations in the amplifier apparatus, and utilized to vary the attenuation in the amplifier--for example, through the use of a voltage controlled attenuator at the amplifier input. At the same time, power limiting control circuitry is incorporated in the apparatus to monitor the power in the transistor output stage, to again vary the attenuation in keeping the output power below a predetermined level. In the usual manner of operation, switching essentially occurs between the AGC mode of operation, and the power limiting mode of operation, depending upon the magnitude of the direct voltage generated from the power transistor output at any instant of time. In these arrangements, the direct voltage is generated by RF diode detector circuits which generate DC voltages proportional to the RF output power of the amplifier.
While such controlled switching has performed adequately well, analysis has shown that the performance deteriorates significantly at very high frequencies. At frequencies, for example, of the two Gigahertz utilized with personal communication services (PCS) phones in a Time Division Multiple Access (TDMA) system, the Base Station employed is generally unable to respond fast enough to protect the output power transistor from destruction during those situations where the input control is suddenly increased--as may be the case with radios, which drive the power amplifier, and which employ dynamic power control (DPC). Even though temperature compensation circuits may be included in an attempt to protect against this where ambient temperature suddenly rises, the operation of the conventional power limiting control circuit has been determined to be of little use in quickly responding when the failure follows from a mistaken belief that the volume setting on the amplifier apparatus is too low and requires increase, when, in fact, the minimal output detected results from a malfunction in the components coupled between the amplifier output and the transmitting antenna. As will be appreciated, continuing to increase the volume setting in those circumstances only serves to destroy the output power transistor.